Nearly every type of RF communication receiver has some kind of Automatic Gain Control (AGC) feature to adjust the gain of the RF chain to an optimal level based on RF signal strength e.g. at the input of the receiver. The AGC is adapted to keep the output signal level substantially constant irrespective of the increase or decrease in the level of the received signal. In fact, at the receiver, the input signal generally varies over a given dynamic range. This may be due to variation of channel conditions or to the source of the received signal moving away from the receiver (e.g., a mobile handset being operated in a fast driven car).
Additionally, components such as an analog-to-digital converter (ADC) are built with an input capability limited by the number of bits at the output (also known as “bitwidth”), hence limiting the dynamic range of input signals acceptable by the receiver. Therefore, without AGC feature, the input dynamic range of the ADC would limit the receiver that would either be saturated with large signals (also known as “clipping”) or be below a tolerable noise level (also known as “jamming”) leading to poor performances.
High and low input signals fed to an RF Digital Front End (DFE) circuit need to be properly measured so that proper gain is applied, for instance, at the input of the ADC. State of the art AGC loops use fixed signal level threshold to assess jamming condition or clipping condition. If a signal level is found below a predetermined low signal threshold level then the gain is set e.g. between its maximum value and a value corresponding to half of the dynamic gain range. On the other hand, if a high signal level is found above a predetermined high signal threshold the gain would be set between its minimum value and half of the dynamic gain range.
Unfortunately, state of the art mechanisms show weaknesses at limit of jamming and clipping since they apply same gain to a range of signals at proximity of these thresholds. Therefore, improper gain, i.e. gain higher than required, may be applied to signals leading to degradation of the Signal-to-Noise Ratio (SNR) downstream in the RF chain, e.g. a RF mixer and/or a Intermediate Frequency (IF) stages, and thus at the input of the demodulator in the baseband (BB) unit.
More importantly, the state of the art mechanism estimates blindly the gain because the AGC has no information of how far the signal saturates the ground floor. Hence the estimated gain may be inducing the signal to saturate or jam.